Biodiversity & Conservation

Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand



Image Sarah Fowler - Maerl bed in Loch Gairloch, Highland. Image width ca 2 m in foreground.
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Distribution map

SS.IGS.Mrl.Phy.HEc recorded (dark blue bullet) and expected (light blue bullet) distribution in Britain and Ireland (see below)

  • EC_Habitats
  • UK_BAP

Recorded distribution in Britain and Ireland

Present at locations within the photic zone all along the west coast of Scotland, including the Western Isles, Orkney, and Shetland. The biotope is almost certainly present round the Isle of Man. Localised occurrences in southern England. Also Strangford Lough in Northern Ireland. The biotope has not yet been officially recorded from Ireland but is expected to be widespread but patchily distributed around the south and west.

Habitat preferences

Temperature range preferences - 10-20 °C.

Water clarity preferences - High clarity / Low turbidity
Very high clarity / Very low turbidity

Limiting Nutrients - Calcium

Other preferences - See additional information.

Additional information

Growth of Phymatolithon calcareum ceases at 5 degrees C and is optimal at 15 degrees C (typically higher than water temperatures found around the British Isles). Growth of Phymatolithon calcareum is impaired at salinities below 24 psu (King & Schramm, 1976) so the species is absent from areas with variable or reduced salinity. Distribution of maerl is dependent on several factors. Living maerl has poor tolerance of desiccation and so is typically found subtidally (Hall-Spencer, 1998). As a photosynthesising organism there is a requirement for light which restricts the species to depths shallower than 32m in the relatively turbid waters of northern Europe (Hall-Spencer, 1998). Some shelter from wave action is required to prevent physical damage, dispersal or burial although some degree of water movement is important to ensure that silt does not smother the maerl bed. King & Schramm, (1982) report that ionic calcium concentration is the main factor affecting growth of maerl in culture experiments rather than salinity per se (although this has not been shown in the field). Uptake of calcium carbonate occurs optimally at 30 psu.

This review can be cited as follows:

Jackson, A. 2006. Phymatolithon calcareum maerl beds with hydroids and echinoderms in deeper infralittoral clean gravel or coarse sand. Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 27/11/2015]. Available from: <>